diff --git a/src/config/plugin-auto-enable.core.test.ts b/src/config/plugin-auto-enable.core.test.ts
index 1f397085157..cd6d49f6598 100644
--- a/src/config/plugin-auto-enable.core.test.ts
+++ b/src/config/plugin-auto-enable.core.test.ts
@@ -228,6 +228,94 @@ describe("applyPluginAutoEnable core", () => {
     expect(result.changes).toContain("codex/gpt-5.4 model configured, enabled automatically.");
   });
 
+  it("does not auto-enable Codex when only the OpenAI plugin is explicitly enabled", () => {
+    const result = applyPluginAutoEnable({
+      config: {
+        plugins: {
+          allow: ["openai"],
+          entries: {
+            openai: { enabled: true },
+          },
+        },
+      },
+      env,
+      manifestRegistry: makeRegistry([
+        { id: "openai", channels: [], providers: ["openai", "openai-codex"] },
+        {
+          id: "codex",
+          channels: [],
+          providers: ["codex"],
+          activation: { onAgentHarnesses: ["codex"] },
+        },
+      ]),
+    });
+
+    expect(result.config.plugins?.entries?.codex).toBeUndefined();
+    expect(result.config.plugins?.allow).toEqual(["openai"]);
+    expect(result.changes).toEqual([]);
+  });
+
+  it("keeps OpenAI Codex OAuth model refs owned by the OpenAI plugin", () => {
+    const result = applyPluginAutoEnable({
+      config: {
+        agents: {
+          defaults: {
+            model: "openai-codex/gpt-5.5",
+          },
+        },
+      },
+      env,
+      manifestRegistry: makeRegistry([
+        { id: "openai", channels: [], providers: ["openai", "openai-codex"] },
+        {
+          id: "codex",
+          channels: [],
+          providers: ["codex"],
+          activation: { onAgentHarnesses: ["codex"] },
+        },
+      ]),
+    });
+
+    expect(result.config.plugins?.entries?.openai?.enabled).toBe(true);
+    expect(result.config.plugins?.entries?.codex).toBeUndefined();
+    expect(result.changes).toEqual([
+      "openai-codex/gpt-5.5 model configured, enabled automatically.",
+    ]);
+  });
+
+  it("auto-enables Codex only for the native Codex harness with OpenAI model refs", () => {
+    const result = applyPluginAutoEnable({
+      config: {
+        agents: {
+          defaults: {
+            model: "openai/gpt-5.5",
+            embeddedHarness: {
+              runtime: "codex",
+              fallback: "none",
+            },
+          },
+        },
+      },
+      env,
+      manifestRegistry: makeRegistry([
+        { id: "openai", channels: [], providers: ["openai", "openai-codex"] },
+        {
+          id: "codex",
+          channels: [],
+          providers: ["codex"],
+          activation: { onAgentHarnesses: ["codex"] },
+        },
+      ]),
+    });
+
+    expect(result.config.plugins?.entries?.openai?.enabled).toBe(true);
+    expect(result.config.plugins?.entries?.codex?.enabled).toBe(true);
+    expect(result.changes).toEqual([
+      "openai/gpt-5.5 model configured, enabled automatically.",
+      "codex agent harness runtime configured, enabled automatically.",
+    ]);
+  });
+
   it("auto-enables an opt-in plugin when an embedded agent harness runtime is configured", () => {
     const result = applyPluginAutoEnable({
       config: {